This invention relates in general to mobile hoists and, in particular, to a straddle-type mobile hoist wherein the lifting load is automatically equalized to obtain optimum load distribution.
More specifically, but without restriction to the particular use which is shown and described, this invention relates to a straddle-type mobile hoist wherein the hoist loading is automatically equalized without requiring any load sensing device for controlling operation of the hoisting mechanism.
While this invention may be utilized with any straddle-type of mobile hoist, for convenience of illustration, the preferred embodiments will be described with reference to use with a sling-rigged boat hoist wherein the lifting load applied through the slings is automatically equalized to obtain optimum load distribution on the hull of a boat being lifted. Such sling-rigged boat hoists are used at harbor facilities to remove boats from the water and carry the boat on land to a storage or repair area. These boats are generally of a size such that they cannot be launched and removed from the water by vehicle drawn trailers or the launching facilities are such that they are not suitable for a launching ramp. These mobile hoists are generally mounted on wheels and provided with a source of power to move over land while carrying the boat in the hoist slings.
In operation, these mobile hoists are moved into a position straddling a slip formed between facing edges of two wharves, or piers, spaced a predetermined distance apart. Movable hoisting slings, which are supported by a structural framework, are lowered into the water and a boat is maneuvered over the slings. The slings are then raised to lift the boat out of the water so that the hoist can carry the boat across the land to its destination.
In the use of such mobile hoists, the loading applied to the boat hull from the lifting slings is frequently uneven, and uneven raising of the hoist slings can result in damage to the boat hull structure. One attempted solution to this problem of uneven loading and uneven raising of the hoist slings has been to use load sensing devices connected to the sling hoisting mechanism so that the lifting hoists operate in response to a differential in the loading applied to the slings. However, such devices add to the cost of the mobile hoist mechanism and are susceptible to malfunctions which cause improper operation of the lifting mechanism and compound the difficulties in attempting to alleviate the problems of uneven loading by the lifting slings.